1. Field of the Invention
The present invention relates to a printing apparatus comprising a printing section for subjecting a board to printing.
2. Background Art
Heretofore, as a printing apparatus for subjecting a board to printing, there has been known a screen printing machine designed to print solder paste onto a workpiece (target object) fixed on a table, through a mask (screen mask), as disclosed, for example, in JP 2006-281680A.
In a screen printing machine disclosed in this publication, at least two error-compensating readable marks are provided at respective positions on an upper surface of a workpiece (target object) holding table. Further, a camera is disposed above the workpiece (target object) holding table in such a manner as to be displaceable parallel to the upper surface of the table. During printing, an image of the two readable marks is picked up by the camera, and a distance between the readable marks is measured from the image to detect a stain of the table (elongation and contraction of the table caused by temperature/humidity changes), so that it becomes possible to perform a strain compensation for the table.
However, in the above conventional screen printing machine, depending on a size or outer shape of the workpiece (target object), one or more of the readable marks are likely to be covered by or hidden behind the workpiece, because the readable marks are provided on the upper surface of the workpiece (target object) holding table. In this case, it is impossible to pick up an image of the marks so as to measure a distance between the marks, at least unless the workpiece is removed from above the readable marks.
Meanwhile, a strain of the table will occur along with a continuous printing operation. Thus, it is desirable that the strain compensation is performed at an appropriate timing, irrespective of the progress or status of the printing operation. However, in the conventional screen printing machine, the strain compensation can be performed only at a limited timing, because an image of the readable marks cannot be reliably picked up unless the workpiece is removed from the table. Thus, there is a problem that the strain compensation timing is extremely restricted.